The popularity of coloring books for adults has exploded in the last year, with an estimated 12 million books sold in 2015 for people looking for ways to relax after work or satisfy their artistic side. In Washington, D.C., one meet-up group has 190 members, and weekly events often have waiting lists. (Jorge Ribas/The Washington Post)

The amateur artists can be found coloring in airport lounges, doctor’s office waiting rooms and while they watch TV shows at home. They are forming coloring meet-up groups at libraries and coffee shops so they can chit-chat as they doodle. Coloring books for adults — a genre once considered little more than a novelty — are suddenly a big business, a bright spot in the financial results of publishers and retailers alike. Nielsen Bookscan estimates that some 12 million were sold in 2015, a dramatic jump from the 1 million sold the previous year.

Whether it is a short-lived fad remains to be seen. The new generation of books are typically filled with intricate black-and-white illustrations that are art themselves. While many find the act of coloring to be a calming distraction from hours spent tapping, swiping and staring at screens, some early adopters aren’t exactly hooked. Several reviewers on Amazon.com found the need to stay in the lines to be anything but soothing.

“Most of the pages are full of pictures that are so small I can hardly see the details to color them, which causes more stress than if I hadn’t tried to color in the first place,” wrote one reviewer of a popular coloring book on Amazon.

While adult coloring book sales have escalated in the United States in the past year, experts say the catalyst for the craze was the work of Scottish author Johanna Basford, whose 2013 title, “Secret Garden: An Inky Treasure Hunt and Colouring Book,” began burning up bestseller lists with its detailed images of topiaries and flowers, and its “Where’s Waldo”-esque challenge to find hidden items in the elaborate illustrations.

Basford and other authors have attracted legions of enthusiasts who are looking to de-stress, who see scrawling away at an image of a tree or an animal as a low-key, low-stakes way to channel imagination or to keep their hands busy while they let their minds wander.

“It’s nostalgic, and it’s a bit old school,” said Mary Amicucci, chief merchandising officer at Barnes & Noble. “It reminds people of their childhood.”

Indeed, Elizabeth Himeles, 26, said she has taken up coloring as a way to tap into some of the creativity she used when she attended arts camp as a kid.

“I don’t have a lot of time in life to do big craft projects, and sometimes I just want to unwind and not do something super active,” Himeles said.

Himeles said it’s not unusual for her to spend up to two hours a week coloring, and sometimes up to four hours in a week when she gets together with a coloring group she organized in the Boston area.

“A lot of the people who come to my meet-ups are really interested in the idea of unplugging and being more mindful,” Himeles said.

It’s not clear whether the rise of adult coloring books has come at the expense of sales in other categories, but the impact of the craze can be seen in various corners of the retail industry: Barnes & Noble has said that strong demand for adult coloring books and artist supplies provided a tail wind to the chain’s total sales in the last three quarters. Walmart, meanwhile, moved in November to add a dedicated four-foot section for adult coloring books in 2,000 of its stores. And Target started carrying adult coloring books in 1,300 stores in August and within months rolled them out to the rest of the chain. Initially, the big-box retailer was carrying only four titles in stores; this month, it’ll be up to 40.

If you look at Amazon’s bestselling books list, which updates hourly, you’re nearly certain to see several adult coloring book titles. (Jeffrey P. Bezos, Amazon’s founder and chief executive, owns The Washington Post.)

“I’ve been in this business for 20 years, and I’ve never seen anything like this,” said Kathleen Schmidt, vice president at Running Press, which hurried to publish four adult coloring books last year when it saw the category gathering momentum.

The popularity of adult coloring books has pushed retailers to move quickly to cater to fans.

Crafting retailer Michaels has expanded its assortment to more than 150 coloring books and promises more this year. Idalia Farrajota, Michaels’s senior vice president of merchandising, said the store has developed exclusive titles to try to keep a leg up on competitors and has sought to expand beyond flora-and-fauna-heavy designs by adding Harry Potter and Star Wars adult coloring books to its lineup.

“It’s been hard to keep up with demand,” Farrajota said.

That the trend came on relatively fast has underscored how quickly and nimbly retailers and others have to move if they want to ride the wave of a trend in the digital era.

Take, for example, the story of Blue Star Coloring, a nascent publishing start-up. Gabe Coeli, its chief creative officer, said the team published its first adult coloring book last March, initially believing it would be something of a placeholder business before it pursued other publishing ventures. That month, it sold just seven copies on Amazon of its first creation, titled “Stress Relieving Patterns.” By April, it had sold 15,000 copies, and by May, the book had rocketed to the top of Amazon’s best-selling book list.

In 2015, the company sold more than 1 million books, including “Stress Relieving Patterns” and subsequent titles the team hustled to assemble when it realized it had a hit on its hands.

“We didn’t realize what we were on the cusp of. We didn’t realize there was this big movement,” Coeli said.

Coloring book enthusiasts meet at Busboys and Poets in the District. (Jorge Ribas/The Washington Post)

Janine Klein of the District said she has long dabbled in coloring as a way to blow off steam since, as a nanny, she often has coloring books within arm’s reach. But she’s been grateful for the new wave of grown-up titles, including one she bought for herself and several friends called “Unicorns are Jerks.”

“It helps to not have to color princesses or Disney characters,” Klein said.

But others are perplexed by grown-ups taking cues from the kindergarten crowd. Robrt Pela, a writer who also curates a contemporary art gallery in Phoenix, Ariz., is among that group.

“I’m a snob. But I’m also an adult, one who remembers when adults relaxed with bourbon, not Crayolas and an outline of My Little Pony,” he wrote in a February article in the Phoenix New Times.

At Barnes & Noble, Walmart and Michaels, executives said the growing interest in adult coloring books has also prompted a surge in sales of a host of related art supplies. Indeed, Nielsen reports that total sales of colored pencils shot up 26.3 percent in 2015, a sharp increase compared to the previous three years, when growth ranged from 1.3 to 7.2 percent.

That pattern is likely why Crayola, the Hallmark Cards-owned giant of kids’ art supplies, moved late last year to launch “Color Escapes,” a line of chicly packaged coloring books and colored pencils that it is aimed squarely at adults.

It seems likely that the retailing and publishing industries will keep looking for ways to capitalize on the coloring craze in 2016. Amicucci said she believes this marks the beginning of a broader trend toward personal expression, and she said Barnes & Noble will be investing in similar categories such as painting, calligraphy and illustration.

Michaels, meanwhile, moved during the holiday season to put out coloring-book-style note cards that Farrajota said you can “personalize and send it to your bestie and wish her a happy birthday. Coloring is not just about the book anymore.”

And Farrajota would know: In her office, she has framed and wall-mounted her own coloring-book illustration of a horse.